A procedure has been developed for the extensive purification of tyrosine hydroxylase from rabbit adrenal glands. This enzyme can be activated by anionic phospholipids, or by exposure to enzymatic phosphorylating conditions. A comparison of the manner by which these two agents activate tyrosine hydroxylase is underway. The information that we have collected thus far indicates that: 1) the activation mechanisms are not additive, i.e., after full activation has been achieved by one technique, subsequent exposure of the enzyme to other manipulations does not result in further stimulation, and 2) each activation procedure results in an irreversible stimulation, as judged by gel filtration and dilution experiments. One of the anionic phospholipids, phosphatidylinositol, can effect either an activation of tyrosine hydroxylate, or an inactivation of this enzyme under specific preincubation conditions. This inactivation appears to be irreversible and phosphatidylinositol can also be used to inactivate tyrosine hydroxylase previously activated by exposure to enzymatic phosphorylating conditions. The conditions that lead to inactivation depend upon time, temperature, pH and buffer composition used in the preincubation. To date, however, our data does not indicate whether this inactivation involves a second enzyme or is the result of a direct interaction between phosphatidylinositol and tyrosine hydroxylase. Further investigation of the mechanism of this inactivation and the nature of the activating conditions is underway.